Public works machine having a safety device for the manoeuvre of its working equipment

ABSTRACT

A loader having a body on which is pivotally mounted a jib system and a bucket and which is operated by fluid motor means is provided with a discharge conduit extending from the bucket raising side of one of the fluid motor means and control valve means to open the conduit to fluid flow whereby movement of the bucket over the loader cab is prevented.

United States Patent [1 1 Bourges Sept. 16, 1975 [54] PUBLIC HAVING A 3,400,634 9/1968 wiflz 91/358 A SAFETY DEVICE FOR THE MANOEUVRE 3,429,471 2/1969 Austin et a1. 214/762 3,670,628 6/1972 Borer et a1 91/358 A OF ITS WORKING EQUIPMENT [75] lnventor: Bernard M. Bourges, Le Plessis Belleville, France [73] Assignee: Societe Anonyme: Poclain, Le Plessis Belleville, France [22] Filed: Feb. 7, 1974 211 App]. No.: 440,581

[30] Foreign Application Priority Data Mar. 9, 1973 France 73.08621 [52] US. Cl 214/762; 91/358 A [51] 1nt. Cl. E02F 3/86 [58] Field of Search 214/762, 764; 91/358 A; 37/DIG. l, DIG. 19

[56] References Cited UNITED STATES PATENTS 3,155.252 11/1964 Muellner et al. 214/762 Primary Examiner-A1brt J. Makay Assistant Examiner-Ross Weaver Attorney, Agent, or Firm-Lewis H. Eslinger; Alvin Sinderbrand [57] ABSTRACT A loader having a body on which is pivotally mounted a jib system and a bucket and which is operated by fluid motor means is provided with a discharge conduit extending from the bucket raising side of one of the fluid motor means and control valve means to open the conduit to fluid flow whereby movement of the bucket over the loader cab is prevented.

5 Claims, 4 Drawing Figures PUBLIC WORKS MACHINE HAVING A SAFETY DEVICE FOR THE MANOEUVRE OF ITS WORKING EQUIPMENT ent loaders have no safety device to prevent such a move.

The present invention provides a loader comprising a body member; means on the body member defining a first pivot axis; a jib system mounted on said body member for pivotal movement about said first pivot axis and comprising at least one jib member; means on said jib system defining a second pivot axis; means for mounting an implement member on said jib system for pivotal movement about said second pivot axis; first fluid motor means for rotating said jib system about said first pivot axis; second fluid motor means for rotating said implement member about said second pivot axis; control valve means responsive to the relative angular positions of said members; means for applying fluid pressure to said fluid motor means; a fluid reservoir, and a discharge conduit extending from one of said fluid motor means to said reservoir via said control valve means, said control valve means being responsive to the relative angular positions of said members to open said conduit to fluid flow when said angular positions enter pre-determined ranges whereby operation of said one of said fluid motor means to raise said implement member is inhibited.

In order that the invention may be fully understood an embodiment thereof will now be described by way of example, with reference to the accompanying drawings in which:

FIG. 1 is an elevation of a loader in accordance with the invention,

and a counter-rod l3, pivoted about axes 14. Rams for adjusting the positions of the various pivoted elements are coupled between these elements. They are respectively:

the ram 15, coupled between the turret 2 and the jib 5 by means of two axes 16,-

-the ram 17 coupled between the jib 5 and the dipperstick 7 by means oftwo axes'l8,

the ram 19 coupled between the dipper-stick 7 and the rocking-lever 12 by means of two axes 20. It will be noted with regard to this ram 19, that it is the pistonrod 21 which is coupled to the rocking-lever 12 and that, consequently, it isthe large chamber 22 of the ram which contains the pressurized fluid whose influence causes the raising of the bucket 9 and the load 11 which it contains.

It will also'be noted that valves 23, 24 and 25 are coupled respectively between the turret 2 and the jib 5, between the jib 5 and the dipper-stick 7 and between the dipper-stick 7 and the bucket 9. For this purpose, the bodies 26, 27 and 28 of said valves are attached respectively to the frame 2, the jib 5 and the dipper-stick 7. Inside the said bodies are slidably mounted mobile slides to which rods 29, 30 and 31 are rigidly attached. The ends of said rods are arranged facing projections 32, 33 and 34 rigidly attached to the jib 5, the dipperstick 7 and the bucket 9 respectively.

FIGS. 2, 3 and 4 are diagrams of the hydraulic feed circuit of the ram coupled between the bucket and the dipper-stick of the loader shown in FIG. 1, in three distinct operating conditions.

The machine described with reference to FIG. 1 consists of an hydraulic shovel fitted with a loader bucket.

and able to function as a loader.

This shovel consists of a chassis with caterpillartracks 1 on which a turret 2 is mounted to rotate about a vertical axis 3. A control-station is arranged on the turret 2 inside a cab 4.

The working equipment of the shovel comprises a firstjib 5 pivoted on the turret 2 about a horizontal axis.

6, a second jib 7 usually called a dipper-stick 7 pivoted on the jib 5 about a horizontal axis 8, and a loader bucket 9 pivoted on the dipper-stick 7 about a horizon I tal axis 10.

The bucket 9 in this instance contains a load of mate- Y FIGS. 2, 3 and 4 show the control circuit of the ram coupled between the bucket 9 and the dipper-stick 7 in three distinct configurations. The elements already described above are referenced by the same numbers, but completed by the indices a, b or c, in FIGS. 2, 3 or 4 respectively, in order to distinguish the different positions occuppied by these elements in said figures.

With reference to FIG. 2, the bucket ram is shown at 19a, its piston-rod being at 21a. Inside the cylinder, its piston 35a defines the chamber of large working section 22a and the chamber 36aof small working section. The piston 35a is then arranged in an intermediate position between the ends (if the ram cylinder. The abovementioned valves are arranged at 23a, 24a and 25a, in series in a same conduit 37 known as the discharge conduit, which is connected on the one hand to the chamber 22a by means of a connection to a conduit 38 connected directly to the said chamber 22a, and on the other hand to a fluid reservoir 39. Each of these valves has two positions:

a first position in which continuity of the conduit 37 is effected at the valve, which position corresponds to a configuration in which the projection corresponding to the valve has pushed the rod of the mobile slide of the said valve; the valves 23a and 24a are placed in their first positions, projections 32a and 33a having pushed rods 29a and 30a respectively;

a second position in which the conduit 37 is interrupted at the valve in question; the valve 25a is thus arranged inthe second position, the projection 34a being out of contact with the rod 31a. The position of the projection 34a obviously relates to the position of the piston35a in the cylinder of the ram 19a, just as the positions of the projections 32a and 33a relate to the relative positions of the jib 5 and the dipper-stick 7, and the jib 5 and the turret 2. It should be noted that a spring 40a, 41a and 42a tends to return the mobile slides of the valves 23a, 24a and 250 respectively into their sec- 0nd positions.

Furthermore, a distributor 43 with at least two positions and in this case three positions, is provided. A pump 44 is connected to the reservoir 39 by its intake conduit 45 and to the distributor 43 by its delivery conduit 46. An auxiliary conduit 47 connects the delivery conduit 46 to the reservoir 39, a calibrated discharge valve 48 being arranged in this conduit 47. It will also be noted that a conduit 49 connects the distributor 43 to the reservoir 39, while a conduit 50 connects the chamber 36a to the distributor 43. The three positions of this distributor correspond:

the first position, to the placing in communication of the conduits 46 and 38 and of the conduits 49 and 50;

the second position, to the placing in communication of the conduits 46 and 49 and to the blocking at the distributor of the conduits 38 and 50;

the third position, to the placing in communication of the conduits 46 and 50 and of the conduits 49 and 38.

Conduits S1 and 52 are connected as a by-pass to the conduits 38 and 50, calibrated discharge valves 53 and 54 being arranged in these conduits 51 and 52 respectively. Similarly, another calibrated valve 55 is arranged between the valve 230 closest to the reservoir 39 and the said reservoir 39. A linkage conduit 56 connects the portion of the conduit 37 between the valve 23a and the calibrated valve 55 to the conduit 50. A non-return valve 57 is arranged in the conduit 56 and allows passage of fluid from conduit 37 to conduit 50. Lastly, a calibrated valve 58 is arranged in the conduit 49.

With reference to FIG. 3, the circuit already described with reference to FIG. 2 is shown in a second configuration. The bucket ram is shown at 19b; its various elements are referenced: 21b for the piston-rod, 35b for the piston and 22b and 36b for its large and small chambers. The valves are shown at 23b, 24b and 25b; their springs are at 40b, 41b and 42b and their rods are at 29b, 30b and 31b, facing projections 32b, 33b and 34b respectively. It will be noted in this respect that these valves are all presently arranged in their first positions, each of said projections having effectively pushed back the rods 29b, 30b and 31b against the action of the springs 40b, 41b and 42b respectively. Moreover, this configuration corresponds to the maximum permitted lift of the bucket 9 and the load 11 which it contains. Furthermore, in this instance the piston 35 b v is arranged substantially at one end of the cylinder of the ram 19b.

Lastly, with reference to FIG. 4, the circuit is shown in a third configuration. The bucket ram is referenced 19c and its various elements are referenced 21c for the piston-rod, 350 for the piston and 22(- and 36c for its large and small chambers. The valves are referenced 23c, 24c and 250; their springs are referenced 40c, 41c and 42c and their rods are shown at 29c; 30c and 310 facing. Projections 32c, 33c and 340 respectively. It will also be noted that the valves 23c and 25c are arranged in their first positions, the projections 32c and 34c having effectively pushed back the rods 29c and 310 against the action of the springs 40c and 420 respectively. This configuration corresponds to the maximum permitted range of the jib 5 relative to the turret 2 and of the bucket 9 relative to the dipper-stick 7, in the direction of raising of the bucket. On the other hand, the valve 240 is arranged in its second position, the projection 330 being out of contact with the rod 300. This configuration corresponds to a position of the dipperstick 7 relative to the jib 5 distinct from the maximum range permitted for these two elements.

Of course, the generalisation of the construction of a machine in accordance with the invention includes the case in which there is only a single jib, the bucket being pivoted directly on the said jib. In this case, there would not be more than two valves and the second jib would be merged with the first jib.

On reading the description of the operation obtained by adoption of the machine which has been described, a better understanding will be obtained of the advantages gained from it relative to manoeuvring safety.

The three configurations shown in FIGS. 2 to 4 and the passage from one of the configurations of FIG. 2 or 4 to that of FIG. 3 will be examined.

Firstly, how does one pass from the configuration of FIG. 2 to that of FIG. 3? The configuration of FIG. 2 corresponds to the maximum permitted range of the jib 5 relative to the turret 2 (jib raised) and of the dipper-stick 7 relative to the jib 5 (dipper-stick arranged substantially in the extension of the jib). On the other hand, the bucket has not yet been entirely lifted erect.

The distributor 43 is then arranged in its first position. The fluid delivered by the pump 44 enters chamber 22a through conduits 46 and 38. The fluid in chamber 36a may return besides to the reservoir 39, via conduits 50 and 49 and through the calibrated valve 58. The pressure of the fluid contained in the chamber 220 on the piston 35a has the effect of pushing this piston 35a and consequently causing the piston-rod 21a to leave the cylinder of the ram 19a. Lifting of the bucket 9 is thus brought about. Towards the end of this manoeuvre the bucket 9 reaches an extreme position beyond which the continuation of its pivoting would risk causing the load 11 of the material contained in this bucket to be tipped onto the drivers cab 4. This extreme position corresponds to the maximum permitted range for the bucket 9.

The circuit has then obtained the configuration of FIG. 3. Moreover, it will be understood, and this fact has been stated many times by users, that when the jib 5, the dipper-stick 7 and the bucket 9 have simultaneously reached their extreme range, the machine must be be left in this configuration, for the material 11 would inevitably be tipped onto the drivers cab 4, even without exceeding the said maximum range permitted for each of the above-mentioned pivoted elements. It is therefore necessary for at least one of the elements to return to a range less than its maximum range.

This manoeuvre takes place automatically due to the valves 23b, 24b and 25b, which have been placed in their first positions. In fact it will be noted in FIG. 3 that the fluid contained in chamber 22b and that possibly still emanating from the pump 44 through the conduit 38, if the distributor 43 has been kept in its first position, can return to the reservoir 39 through the conduit 37 and the three above-mentioned valves. As, in addition, the weight P of the bucket 9 and of the load 11 of material permanently tends to return the piston-rod 21b into the cylinder of the ram 19b, the fluid contained in the chamber 22!) is effectively pushed out of this chamber by the piston 35b. The bucket 9 therefore pivots in the opposite direction to previously, which finally eliminates the risk of the load of material 11 being 7 accidently tipped onto the drivers cab 4.

It will also be mentioned that in the case of a machine so constructed that the vertical passing through the centre of gravity of the bucket-material assembly is situated on the other side of the axis 10, the -opening" command of the bucket is actuated to prevent accidental tipping. All that is necessary to do this is to supply the chamber 36a of the ram simultaneously as the chamber 220 is placed in communication with the rescrvoir, in known manner via hydraulic or electrohydraulic valves. This modification is considered as a mechanical equivalent forming part of the presentinvention.

As regards the embodiment shown, it will be noted that as a rule the driver of the machine has kept the distributor 43 in its first position. In this position, the chamber 36b is connected to the reservoir 39 by the conduits 50 and 49. The return motion of the pistonrod 21b into the ram cylinder 19b results in a deficiency of fluid in the chamber 3612, which deficiency of fluid cannot with certainty be made good by the fluid contained in the reservoir 39, which is not able to be conveyed to the said chamber. However, it will be noted that the fluid delivered from the chamber 22b meets with a slight resistance in the conduit 37 at the calibrated valve 55 and consequently tends to be directed through the conduit 56 to the conduit 50, via nonreturn valve 57. As in addition another slight resistance has been formed in the conduit 49 at the calibrated valve 58, the fluid delivered by the piston 35b from the chamber 22b is preferentially directed through the conduits 37, 56 and 50 to the chamber 36b. Taking into account the fact that the chamber 22b has a greater working section than the chamber 36b, the added volume of fluid in the chamber 36b exceeds the volume deficiency caused by the displacement of the piston 35b. Cavitation in the chamber 36b is thus avoided. The surplus volume which cannot enter the said chamber returns to the reservoir 39 through either of the two calibrated valves 55 or 58.

The driver finally notices at a given moment that although he is commanding the maximum raising of the bucket 9, the latter is no longer rising. He then shifts the distributor into its second position. In this position it may happen that, due to a shock on the bucket 9, in one direction or the other, the pressure increases anormally in one of the chambers 22b or 36b. It should also be noted that an anormally great resistance of the material opposing the penetration of the bucket on loading of the said bucket might also cause a same excess pressure. It is for the occurance of such excess pressures that the calibrated discharge valves 53 and 54 have been provided. which limit the pressure in the chambers 22b and 36b to the maximum permitted values beyond which damage to the machine would be expected.

At this stage the tasks of the various calibrated valves 48, 53, 54, 55 and 58 should be distinguished. As observed above, the valves 55 and 58 act principally as fluid check-valves. Their setting is in fact fairly low, for example of the order of bar. On the other hand the valves 48, 53 and 54 act as discharge valves and have a safety function: to limit the pressure of the fluid in certain chambers. Of course. they must allow the action of the pressurized fluid under normal conditions of use. For this reason their setting is high, of the order of 350 bar for example.

Passage from the configuration of FIG. 4 to that of FIG. 3 will now be examined.

The configuration of FIG. 4 corresponds to the maximum permitted range of the jib 5 relative to the turret 2 and of the bucket 9 relative to the dipper-stick 7. Moreover, the distributor 43 is arranged in its second position. On the other hand, the dipper-stick 7 has not reached its maximum possible range relative to the jib 5.

The ram 17 is then operated so as to raise the dipperstick 7 relative to the jib until the maximum range of these two elements is reached. The configuration of FIG. 3 has then been obtained, the distributor 43 still being kept in its second position. As mentioned above, the weight of the load 11 and the bucket 9 has the effect of pushing the fluid contained in the chamber 22b out of this chamber. The conduits 38 and are blocked at the distributor 43. However, just as before, the fluid contained in the chamber 22b is directed to the chamber 36b to force-fill it and avoid cavitation. Here again the calibrated valve fulfils the retaining function which has fallen to it. I

It will be appreciated that, in the machine which has been described, as soon as the maximum ranges'have allbeen reached at a given moment, the safety of the machine in regard to accidental tipping is insured by the valves 23b, 24b and 25b in their first respective positions. It is sufficient for only one of the said valves not to have been arranged in its first position for the safety device not to operate. This is desirable, as in effect only the concordance of the maximum ranges gives rise to a danger of tipping and there is no call to operate the safety device in configurations other than the single configuration in which all the ranges are maximum.

It will also be noted that when the maximum range of one element relative to another, of the jib 5 relative to the turret 2 for example. is reached, the corresponding projection (32a in this instance) is effectively bearing on the rod 29a of the corresponding valve 23a, but that also the two said elements (jib 5 and turret 2) are then in mechanical abutment one against the other. The maximum range is effectively the greatest range mechanically possible.

Lastly, if the application of the safety device to controlling the supply of the ram coupled to the bucket 9 has been given as an example, it obviously remains possible and in accordance with the present invention to apply a similar safety device not to the ram coupled to the bucket but to either of the other rams 15 or 17, in order of course that the vertical passing through the centre of gravity of the pivoting mass in question should remain permanently on the same side of the pivot axis of this mass.

Moreover, the invention is not limited to the description which has been given, but on the contrary covers all modifications which could be made to it without departing from its scope or spirit.

What is claimed is:

l. A loader comprising a body member; means on the body member defining a first pivot axis; a jib system mounted on said body member for pivotal movement about said first pivot axis and comprising at least one jib member; means on said jib system defining a second pivot axis; means for mounting an implement member on said jib system for pivotal movement about said second pivot axis; first fluid motor means for rotating said jib system about said first pivot axis; second fluid motor means for rotating said implement member about said second pivot axis; control valve means responsive to the relative angular positions of said members; means for applying fluid pressure to said fluid motor means; a fluid reservoir; and a discharge conduit extending from one of said fluid motor means to said reservoir via said control valve means, said control valve means being responsive to the relative angular positions of said implement with respect to said jib system about said second pivot axis and said jib member with respect to said body member about said first pivot axis to open said conduit to fluid flow when said angular positions enter pre-determined ranges, whereby operation of said one of said fluid motor means to raise said implement member is inhibited.

2. A loader in accordance with claim 1, wherein fluid means to which the discharge conduit is connected is of the double-acting type and has two chambers, a first chamber to which the discharge conduit is connected, and to which is also connected a first conduit connectable to the source of pressurized fluid, and a second chamber, which is connected to a second conduit connectable to the discharge reservoir, and including a distributor arranged in the first and second conduits, and having a first position producing continuity of the said first and second conduits at that point, and a second position in which the said first and second conduits connected to the two chambers of the said fluid motor means are blocked, a calibrated discharge valve in the discharge conduit between the control valve means and the reservoir, a linkage conduit connecting the discharge conduit between the said control valve means and the said calibrated discharge valve to the portion of the second conduit between the second chamber of the fluid motor means and the distributor, and a nonreturn valve which is arranged in the said linkage conduit to allow passage of fluid from the discharge conduit to the second conduit and to said second chamber, to prevent cavitation in said chamber.

3. A loader in accordance with claim 1 wherein the second fluid motor means consists of a ram and the discharge conduit being connected-to that chamber of the said ram which is supplied with pressurized fluid on raising of the implement member.

4. A loader comprising a body member, a jib system pivotally mounted on said body member and including at least one jib member pivotally mounted on said body member about a first pivot axis; an implement member pivotally mounted on said jib system about a second pivot axis, first fluid motor means for rotating said jib member about first pivot axis with respect to said body member; second fluid motor means for rotating said implement member about said second pivot axis with respect to said jib system; means including a fluid reservoir. for supplying fluid under pressure to said fluid motor means; a fluid discharge conduit operatively connected between one of said first and second fluid motor means and said reservoir to allow discharge of fluid therethrough; first and second control valve means located in said discharge conduit and each having two positions respectively blocking and opening the discharge conduit; said control valve means being respectively operatively associated with said implement member and said jib member with said first valve means being responsive to the relative angular position of the implement member to the jib system to open said discharge conduit at a predetermined angular position therebetween and said second valve means being responsive to the relative angular position of the jib member to the body member to open said discharge conduit at a predetermined angular position therebetween whereby when both said implement member and said jib member are at said predetermined angular positions said first and second control valve means open said discharge conduit to fluid flow from said one of said fluid motor means to said reservoir thereby inhibiting said one of said fluid motor means from operation.

5. A loader as defined in claim 4 wherein said one of said fluid motor means comprises a double acting ram having a piston therein separating the ram into first and second fluid chambers, said first fluid chamber being connected to said discharge conduit; a first conduit connected to said first chamber and to said discharge conduit; a second conduit connected to said second chamber; and a distributor valve operatively connected to said first and second conduits for selectively connecting the first and second conduits to said means for supplying fluid under pressure and said reservoir; said distributor valve having at least first and second positions and means for connecting said first and second conduits to said fluid pressure means and said reservoir respectively in the first position thereof and for blocking fluid flow in said first and second conduits in the second position thereof; a calibrated discharge valve in said discharge conduit downstream of said control valve means; a connecting conduit in fluid communication between said second conduit; between said second chamber and said distributor valve, and said discharge conduit at a position downstream of said control valve means and upstream of said calibrated valve; and nonreturn valve means in said connecting conduit for allowing fluid in said discharge to pass to said second conduit and into said second chamber to prevent cavitation therein 

1. A loader comprising a body member; means on the body member defining a first pivot axis; a jib system mounted on said body member for pivotal movement about said first pivot axis and comprising at least one jib member; means on said jib system defining a second pivot axis; means for mounting an implement member on said jib system for pivotal movement about said second pivot axis; first fluid motor means for rotating said jib system about said first pivot axis; second fluid motor means for rotating said implement member about said second pivot axis; control valve means responsive to the relative angular positions of said members; means for applying fluid pressure to said fluid motor means; a fluid reservoir; and a discharge conduit extending from one of said fluid motor means to said reservoir via said control valve means, said control valve means being responsive to the relative angular positions of said implement with respect to said jib system about said second pivot axis and said jib member with respect to said body member about said first pivot axis to open said conduit to fluid flow when said angular positions enter pre-determined ranges, whereby operation of said one of said fluid motor means to raise said implement member is inhibited.
 2. A loader in accordance with claim 1, wherein fluid means to which the discharge conduit is connected is of the double-acting type and has two chambers, a first chamber to which the discharge conduit is connected, and to which is also connected a first conduit connectable to the source of pressurized fluid, and a second chamber, which is connected to a second conduit connectable to the discharge reservoir, and including a distributor arranged in the first and second conduits, and having a first position producing continuity of the said first and second conduits at that point, and a second position in which the said first and second conduits connected to the two chambers of the said fluid motor means are blocked, a calibrated discharge valve in the discharge conduit between the control valve means and the reservoir, a linkage conduit connecting the discharge conduit between the said control valve means and the said calibrated discharge valve to the portion of the seCond conduit between the second chamber of the fluid motor means and the distributor, and a non-return valve which is arranged in the said linkage conduit to allow passage of fluid from the discharge conduit to the second conduit and to said second chamber, to prevent cavitation in said chamber.
 3. A loader in accordance with claim 1 wherein the second fluid motor means consists of a ram and the discharge conduit being connected to that chamber of the said ram which is supplied with pressurized fluid on raising of the implement member.
 4. A loader comprising a body member, a jib system pivotally mounted on said body member and including at least one jib member pivotally mounted on said body member about a first pivot axis; an implement member pivotally mounted on said jib system about a second pivot axis, first fluid motor means for rotating said jib member about first pivot axis with respect to said body member; second fluid motor means for rotating said implement member about said second pivot axis with respect to said jib system; means, including a fluid reservoir, for supplying fluid under pressure to said fluid motor means; a fluid discharge conduit operatively connected between one of said first and second fluid motor means and said reservoir to allow discharge of fluid therethrough; first and second control valve means located in said discharge conduit and each having two positions respectively blocking and opening the discharge conduit; said control valve means being respectively operatively associated with said implement member and said jib member with said first valve means being responsive to the relative angular position of the implement member to the jib system to open said discharge conduit at a predetermined angular position therebetween and said second valve means being responsive to the relative angular position of the jib member to the body member to open said discharge conduit at a predetermined angular position therebetween whereby when both said implement member and said jib member are at said predetermined angular positions said first and second control valve means open said discharge conduit to fluid flow from said one of said fluid motor means to said reservoir thereby inhibiting said one of said fluid motor means from operation.
 5. A loader as defined in claim 4 wherein said one of said fluid motor means comprises a double acting ram having a piston therein separating the ram into first and second fluid chambers, said first fluid chamber being connected to said discharge conduit; a first conduit connected to said first chamber and to said discharge conduit; a second conduit connected to said second chamber; and a distributor valve operatively connected to said first and second conduits for selectively connecting the first and second conduits to said means for supplying fluid under pressure and said reservoir; said distributor valve having at least first and second positions and means for connecting said first and second conduits to said fluid pressure means and said reservoir respectively in the first position thereof and for blocking fluid flow in said first and second conduits in the second position thereof; a calibrated discharge valve in said discharge conduit downstream of said control valve means; a connecting conduit in fluid communication between said second conduit; between said second chamber and said distributor valve, and said discharge conduit at a position downstream of said control valve means and upstream of said calibrated valve; and nonreturn valve means in said connecting conduit for allowing fluid in said discharge to pass to said second conduit and into said second chamber to prevent cavitation therein. 